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LDC1612: Drive current measurement issue

Cherry Zhou
Mastermind 22235 points
Part Number: LDC1612

Hi Team,

The customer would like to use a PCB coil as the LDC1612 sensor to measure the distance of the PCB coil to a silver-plated mirror.

Issue:

During the movement of the PCB coil (4 mm-20 mm), it is not possible to measure the entire distance with one drive current, or if only one drive current is used, the PCB is too close or too far away from the silver-plated mirrors with amplitude errors.

PCB parameters: PCB diameter up to 26mm due to size constraints (all other parameters can be varied)

Measurement range (PCB coil approximately 4 mm from silver plating due to mirror and assembly): 4 mm-20 mm.

The PCB coil parameters currently designed are as follows:

At the same time, to limit the orientation of the PCB magnetic field, a round ferrite is placed on the back of the PCB coil, but it also results in a reduction in the overall resonant frequency.

1) Can the coil be redesigned such that the entire process uses only one drive current to complete the measurement?

2) If different drive currents are used throughout the measurement phase, it is obvious that the measurement results are not available, that is, the conversion results are jumping. Is there a way to solve this problem?

3) Would it be possible to disregard the amplitude error that occurs? However, experiments have shown that when amplitude errors occur, a bad result can be apparent.

Could you help check this case? Thanks.

Best Regards,

Cherry

  • +1
    TI__Mastermind 18460 points

    Hello Cherry, 

    Changing the coil to only use one drive current would require some trial and error and may not be completely possible. You could try altering the parallel resistance of the coil by adding a resistor to the LC tank and seeing if it decreases the dynamic range of your sensor amplitude for your target interaction. 

    The reason the results jump when you change IDRIVE settings is because it changes the amplitude of the sensor oscillation which slightly impacts the sensor frequency. There is no way to remove this result from the changes in IDRIVE other than some form of firmware processing of the data if you know when the IDRIVE shift occurs and how much it impacts your system each time. 

    For the amplitude error, these occur when the sensor amplitude is outside of the 1.2V to 1.8V range. With a sensor amplitude below 1.2V, the SNR of the measurement is impacted and you may not be able to get as much resolution out of the device. If the sensor amplitude is below 0.6V, then there is a chance the sensor oscillation will no longer be stable and the LDC will be unable to take measurements. When the sensor amplitude is above 1.8V, there is a chance you get a sensor frequency shift as the internal ESD clamp activates and there can be greater drift of the measurement across temperature. In general, it is better to be below1.8V for part of the measurement than it is to be above 1.2V. 

    When you say you get a bad result, what is your sensor amplitude? Are you only getting errors on one end of your target motion or both? 

    Also, for more information on the IDRIVE setting, please see the Setting LDC1312/4, LDC1612/4, and LDC1101 Sensor Drive Configuration app note. 

    Best Regards, 

    Justin Beigel